Papers by Author: Yu Jie Liu

Abstract: This paper reports the fabrication of carbon nanotubes (CNTs) /hydroxyapatite (HA) composites via in-situ process in a chemical vapor deposition (CVD) system and the effect of HA matrix on the growth of CNTs. When the HA matrix is fabricated by a chemical coprecipitation method, deficient HA (D-HA) crystals in nano size have been observed in these composites. There are about 20% weight percent of multi-walled CNTs with a mean diameter of 40~60 nm. When the matrix used as catalyst for CNTs growth without HA in the same way, no CNTs but solid carbon fibers in submicrometer scale have been produced in these composites. Only Fe and MgO crystals can be observed in these composites. When the HA matrix is fabricated by a physical mixing with the presence of D-HA crystals, hollow CNTs with the diameter of 180~210 nm are also produced in these composites. Fe and MgO besides P₂O₅, D-HA and C crystals can be observed in the composites.

Abstract: Uniaxial ratchetting-fatigue interaction of tempered 42CrMo alloy steel was observed by various cyclic stressing tests at room temperature. The ratchetting deformation and low cycle fatigue (LCF) property of the material as well as their interaction occurred in cyclic stressing were discussed. It is shown that progressive ratchetting deformation causes the decrease of fatigue life, and the fatigue life of the material depends greatly upon the applied mean stress, stress amplitude, maximum stress and stress ratio. Since tempered 42CrMo steel presents significant cyclic softening feature, a tertiary ratchetting is observed. Based on the experimental results, a simple and reasonable failure model convenient to engineering application was constructed to predict the fatigue life of the material in uniaxial cyclic stressing. The basic variables of the model are maximum stress and stress ratio, and the effect of cyclic softening feature on the ratcheting-fatigue interaction is also included in the model by introducing a new variable. It is shown that the predicted lives are in fairly good agreement with the experimental ones.

Abstract: The uniaxial/multiaxial cyclic deformation behaviors of SiCp/6061Al alloy composites with various particulate volume fractions were studied by uniaxial and multiaxial cyclic straining or stressing tests at room temperature. The cyclic softening/hardening features and ratcheting behaviors of T6-treated composites and un-reinforced matrix were discussed in different loading conditions. It is shown that the ratcheting also occurs in the composites under uniaxial and multiaxial asymmetrical cyclic stressing, and the ratcheting strain increases with stress amplitude and mean stress; however, the addition of SiC particulates into the matrix increases the resistance of the composite to ratcheting. The ratcheting depends greatly on the shapes of loading paths and mainly occurs in the direction of non-zero mean stress.

Abstract: The cyclic deformation behaviors of 42CrMo steel with different heat treatments were observed by uniaxial cyclic straining and stressing tests at room temperature. The cyclic softening/ hardening features of the tempered or annealed 42CrMo steel and their effects on the uniaxial ratcheting produced in asymmetrical cyclic stressing were discussed. It is concluded that the tempered 42CrMo steel shows significantly cyclic softening feature, but the annealed one is cyclic stabilizing. Different ratcheting behaviors are also observed. For the tempered 42CrMo steel, a special tertiary ratcheting behavior is observed and the previous cyclic straining greatly accelerates the evolution of ratcheting strain in subsequent cyclic stressing. In contrast, the annealed sample presents a stabilized ratcheting with nearly constant ratcheting strain rate after certain cycles, and the previous cyclic straining slightly influences the ratcheting in subsequent cyclic stressing.

Abstract: The cyclic deformation behavior of 6061Al-T6 alloy was studied by uniaxial cyclic straining or stressing tests at room and high temperatures. The cyclic softening/hardening feature of the material and its effect on the uniaxial ratcheting occurred under asymmetrical cyclic stressing were discussed. The cyclic deformation behaviors of the material presented at various loading rates and with certain peak and/or valley strain or stress holds were addressed. It is concluded from the experimental results that 6061Al-T6 alloy presents slightly cyclic softening feature, and its cyclic deformation is time-dependent even at room temperature. The ratcheting strain produced at lower stressing rate and with longer hold-time is apparently larger than that at higher stressing rate and with fewer hold-time. Some significant conclusions useful to construct the constitutive model to describe such cyclic deformation behaviors are obtained.

Abstract: Based on the experimental results of uniaxial time-dependent ratcheting behavior of SS304 stainless steel at room temperature and 973K, three kinds of time-dependent constitutive models were employed to describe such time-dependent ratcheting by using the Ohno-Abdel-Karim kinematic hardening rule, i.e., a unified viscoplastic model, a creep-plasticity superposition model and a creep-viscoplasticity superposition model. The capabilities of such models to describe the time-dependent ratcheting were discussed by comparing with the corresponding experimental results. It is shown that the unified viscoplastic model cannot provide reasonable simulation to the time-dependent ratcheting, especially to those with certain peak/valley stress hold and at 973K; the proposed creep-plasticity superposition model is reasonable when the creep is a dominant factor of the deformation, however, it cannot provide a reasonable description when the creep is weak; the creep-viscoplastic superposition model is reasonable not only at room temperature but also at high temperature.